Current practices for baseline studies of sites to be developed for mining include surface grab sampling of sediments in aquatic receiving environments. In contrast, vertical sediment coring is a
universal tool in paleolimnological research. This study evaluates the effectiveness of sediment grab sampling versus sediment coring for environmental risk assessment of metal mining. The former Aldermac mine (Cu, Zn, Au and Ag), 15 km west of Rouyn
- Noranda in Abitibi, Quebec, operated from 1932 - 1943 and discharged acid mine drainage into the watershed downstream. The study area is representative of both a common mineral deposit type and historical mining practices. Surveys of bulk sediment
grab samples (2011 - 13) were done using a Ponar® sampler that penetrated to a depth of approximately 5 - 10 cm at 34 sampling sites. Co - located sediment cores, 30 - 45 cm long, were collected using a modified 10 - cm diameter gravity corer at 33
sites and were subsampled at 0.5 to 5 - cm depth intervals. Geochemical results from quick regional grab sampling provided estimates of baseline metal concentrations in surface sediments, contaminant sources, and the spatial extent of metal
contamination. Sediment samples retrieved by longer cores produced estimates of both spatial and temporal factors: naturally - occurring metal concentrations (pre - industrial background), current metal concentrations, contaminant sources, and the
duration and spatial extent of contamination. Although regional grab samples provide more sample material and are faster and simpler to collect than gravity coring, the results are imprecise because they are bulk data integrated over roughly 5 - 10
cm of depth; they also lack a time dimension. Coring, core extrusion, and subsampling at distinct depth intervals provide chemical stratigraphy of metal deposition. Coring also affords potential for more targeted data to fingerprint contaminant
sources, assess diagenetic metal mobility, and determine stability of metal - bearing phases at discrete sediment horizons. These research-grade investigations can be optimized for practical use in environmental risk assessment. Bulk grab sampling
offers a first approximation of regional metal distributions in shallow sediments. However, when more detailed investigation is warranted under environmental risk assessment regulations, sediment coring can not only provide estimates of the true
ranges of naturally - occurring metal concentrations (natural background), but also measurements and timelines of metal increases from industrial activities.

Summary

(Plain Language Summary, not published)Different methods of aquatic sediment sampling downstream from a known contaminated site (the former Aldermac mine in Abitibi, Quebec) are presented to
demonstrate options for environmental risk assessment of metal mining. Shallow bulk sediment samples can be taken quickly and the results can be used to estimate baseline conditions (before new development), contaminant sources, and the spatial
extent of contamination, although the method generates imprecise data without time scales. When further investigation is warranted, sediment coring provides results that can determine accumulated metal contamination over time, higher precision, and
more targeted data (e.g., to identify contaminant sources and metal mobility).